The constant push in the integrated circuit industry to generate smaller features for next-generation computer chips has caused great interest in improved lithographic techniques. Traditionally, however, it was believed that for a given wavelength used to write the patterns, there was a limit to the feature size given by the Rayleigh criterion. While for many years the techniques faced more practical limitations than the physical limitation imposed by Rayleigh's criterion, technology has now reached the level to meet this limit. Thus, it is commonly viewed that the primary option to reduce feature sizes further involves going to shorter and shorter wavelengths. As optical lithography has already approached the lower wavelength limits, this would leave the next alternative seemingly to switch to e-beam or x-ray lithography, both of which are extremely costly and have many practical drawbacks at this time. Therefore, there is great interest in developing new techniques to bypass the Rayleigh criterion, allowing the integrated circuit industry to continue to make major advances without the costly need to abandon optical lithography on which it is always depended.
In the past several years, such techniques began to be developed, all relying on materials that absorb multiple photons at a time, rather than one at a time as traditionally used. While some of the earliest proposals were classical, most of the attention has been given to quantum lithography. Although it is interesting due to the special properties that it could provide, there are a number of disadvantages and limitations with quantum lithography, especially with respect to its future use in the circuit industry.
In an article co-authored by the present applicant, namely, S. J. Bentley and R. W. Boyd, “Nonlinear optical lithography for ultra-high sub-Rayleigh solution,” Optics Express 12, 5735 (2004), a one-dimensional, classical, nonlinear interferometric lithographic system is described. The primary idea is to interfere two beams on an N-photon absorbing lithographic substrate. This is repeated M times (with M≦N), each time increasing the relative phase between the two beams by 2π/M. This results in canceling the undesired low-resolution features and leaving only features with a resolution M-times better than allowed by the Rayleigh criterion. One initial concern with all of the classical techniques, such as the one described in this article, is the resulting pattern visibility. If one lets M=N to achieve the maximum possible resolution enhancement, the visibility quickly degrades beyond a useful level for increasing N. However, as long as N is significantly larger than M, a high visibility is always possible.
The disclosed technique in this reference is limited to one-dimensional patterns and for practical considerations, the technique is limited to very simple 1-D patterns. Thus, there are a number of disadvantages and practical limitations that prevent this technique from having wide-spread success.
It is therefore, desirable to have a simple yet effective process for writing an arbitrary two-dimensional pattern using nonlinear, interferometric lithography techniques. The embodiments described herein satisfy these and other needs.